The present invention relates to a four-port network for separating signals comprised of two doubly polarized frequency bands for an antenna feeder system in directional or satellite radio operation. More particularly, the present invention relates to such a network which includes a polarization converter for converting a linear polarization to a circular polarization and vice versa and designed for the lowest possible inherent ellipticity in the lower of the two frequency bands is connected ahead of a symmetrical polarization filter for the lower frequency band, and a second polarization converter is connected between this polarization filter and a futher polarization filter for the upper frequency band, and wherein the second polarization converter provides compensation for the remaining ellipticity in the upper frequency band.
Four-port networks are used in antenna systems, for example in satellite radio, for the separation of signals when the system is operated with double frequency utilization, where the one frequency band is provided for an up-link or transmission connection and the other frequency band is provided for a down-link or receive connection. Each frequency band has two associated linearly or circularly polarized (rotating clockwise, counterclockwise) signals. Circular polarization is used in the radio art for data transmission whenever parallel alignment of the polarization of the receiving antenna with the polarization of the receiving field intensity is not assured. In this connection, the time variable, nonreciprocal rotation of the polarization plane in satellite transmissions is a rotation which is effected due to the coaction of free electrons with the earth's magnetic field in the ionosphere, particularly at frequencies below 10 GHz.
If unequal transmission properties of the field components occur in the four-port network, the circular polarization changes to an elliptical polarization. The lower the ellipticity, i.e. the ratio of major to minor ellipse axis, the better is the polarization decoupling.
German Offenlegungsschrift No. DE-OS 27 03 878, laid open Aug. 8th, 1978, discloses a four-port network, in which, as shown in FIG. 1, the respective polarizations of two linearly polarized signals are initially separated by means of a polarization filter 1 and thereafter a pair of filters 2 and 3, are provided which filter the polarization components of lower frequency band (4 GHz) out of each polarization signal and transmit polarization components of the upper frequency band (6 GHz). In order to produce right or clockwise and left or counterclockwise circular signals in the respective lower and upper frequency bands, the signals in the lower and upper frequency bands, which have been separated into their respective linear polarization directions, are fed to respective 3 dB couplers 4 and 5 which effect a conversion into right or clockwise (rz) and left or counterclockwise (1z) circular polarization components. To achieve an ideal circular polarization, the signal paths associated with each 3 dB coupler 4 and 5 must have precisely the same propagation conditions. However, in reality this cannot be accomplished so that the circular polarization always exhibits some ellipticity.
The publication by the applicant G/unther Morz, "Analyse und Synthese von elektromagnetischen Wellenfeldern in Reflektorantennen mit Hlife von Mehrtyp-Wellenleitern" [Analysis and Synthesis of Electromagnetic Wavefields in Reflector Antennas with the Aid of Multiple-Type Waveguides], Dissertation, D82, RWTH-Aachen, Germany (1978), pages 75-81 and particularly pages 80 and 81, discloses a four-port network which provides measures for minimizing the ellipticity of the polarization. This four-port network as shown in FIG. 2, includes a first polarization converter 6 which receives linearly polarized signals from the antenna feedhorn (not shown) and converts them to circularly polarized signals or converts circularly polarized signals to linearly polarized signals and transmits them to the antenna feedhorn. This polarization converter 6, in the receiving direction, is followed by a polarization filter 7 which filters out or separates the circularly polarized clockwise (rz) and counterclockwise (1z) signals in the lower frequency band (e.g. 4 GHz) from the output of converter 6 and permits the signals in the upper frequency band (e.g. 6 GHz) to pass. The polarization filter 7 is followed in turn, by a second polarization converter 8 for the upper frequency band and a further polarization filter 9 for separating the clockwise and counterclockwise circular signals rz and 1z respectively in the upper frequency band. A polarization converter which is capable of converting a linearly polarized wave into a circularly polarized wave is disclosed, for example, in applicant's U.S. Pat. No. 3,758,882, issued Sept. 11th, 1973. According to the above-identified publication the first polarization converter 6 is to be designed to have the lowest possible inherent ellipticity (which is dependent upon the frequency) in the lower frequency band, whereas compensation for the remaining ellipticity in the upper frequency band is to be provided by the second polarization converter 8. With this arrangement, it is possible to provide separate minimization of the inherent ellipticity for each frequency band. If only one polarization converter were used, it would simultaneously have to be optimized for both frequency bands and consequently the ellipticity could not be reduced as far in either frequency band.